1 . Field of the Invention
A marine seismic acoustic signal generator is installed in a small, substantially zero-draft marine support vehicle that is usually towed behind a service ship in a body of water. In operation, the entire vehicle becomes an acoustic wavefield generator.
2 . Discussion of Related Art
As is well known to geophysicists, a sound source, at or near the surface of the earth, is caused periodically to inject an acoustic wavefield into the earth at each of a plurality of regularly-spaced survey stations. The wavefield radiates in all directions to insonify the subsurface earth formations whence it is reflected back to the surface to be received by seismic sensors (receivers). The seismic sensors are located at designated stations at or near the surface of the earth. The seismic sensors convert the mechanical earth motions (or water-pressure variations in a marine environment), due to the reflected wavefield, to electrical signals. The resulting electrical signals are transmitted over a signal-transmission link of any desired type, to instrumentation, usually digital, where the seismic data signals are archivally stored for later processing. The travel-time lapse between the emission of a wavefield by a source and the reception of the resulting sequence of reflected wavefields by a receiver, is a measure of the depths of the earth formations from which the respective wavefields were reflected.
The seismic survey stations of a 3-D survey are preferably distributed in a regular grid over an area of interest with inter-station spacings on the order of 25 meters. The processed seismic data associated with a single receiver location are customarily presented as a one-dimensional time scale recording displaying rock layer reflection amplitudes as a function of two-way wavefield travel time. A plurality of seismic traces from a plurality of receivers distributed sequentially along a line of survey may be formatted side-by-side to form a model of a cross section of the earth (two-dimensional tomography). Seismic sections from a plurality of intersecting lines of survey distributed over an area of interest, provide three-dimensional tomography.
At sea, in deep water, the sensors are mounted in one or more streamer cables and towed through the water by a cable ship. In relatively shallow water, the sensors and their connecting cables are laid on the sea floor from a service boat. Alternatively, in shallow water and the transition zone (the surf), the sensors may be associated with instrumented buoys. All of these techniques are well known to the art and need not be dwelt upon further.
In marine operation, a service (shooting) ship tows an acoustic source such as an array of air guns, a chirp signal generator or other device through the water over the region to be surveyed. As it visits each survey station, the source is triggered to generate a desired wavefield. One such device is disclosed in U.S. patent application Ser. No. 3,384,868, issued May 21, 1968 to G. Brown et al., which is incorporated herein by reference. This is an apparatus for the generation of seismic energy waves within a water medium which consists of a rigid frame member and suspension attachments, the frame member movably securing a piston plate between upper and lower sealed bellows which are alternately pressurized by reciprocal oil flow to vibrate the piston plate relative to the frame member, the apparatus also including the employ of additional bellows which is sealed between the piston plate and frame member to receive varying air pressure to thereby maintain static-pressure balance.
A somewhat similar device is taught by U.S. Pat. No. 3,329,930, issued Jul. 4, 1967 to Jimmy R. Cole et al. This source is a submerged hydraulic vibrator having an inverted bell-shaped body member closed on the bottom by a flexible plate. An hydraulic servo valve, mounted in a cylindrical housing secured to the body member, vibrates a piston that is coupled to the flexible bottom plate. The assembly is designed to be towed through the water, imparting vibratory signals thereto at selected survey stations.
The drag produced by the towed sources necessarily slows down the shooting ship, reducing the production rate of stations occupied per unit time and is therefore costly. It would appear to be useful to incorporate the acoustic source into the structure of the shooting ship itself in a streamlined configuration.
One such system was disclosed in U.S. Pat. No. 3,401,660, issued Sep. 17, 1968 to Booth B. Strange et al. for a Seismic Ship. Here, wells on the order of eight feet in diameter extend from the working deck through the ship's bottom, opening into the water. By means of a crane, a suitable sound source is lowered into the well flush with the hull, but immersed in the water. Either a chirp-signal generator or an impulse-type source was used.
The disadvantage of that particular arrangement was the need for totally-enclosed, sealed sound sources and the wells, open to the sea penetrating the ship's hull. The devices were unwieldy, had a tendency to protrude beneath the hull and were difficult to service during active operations by reason of their total immersion. Furthermore, each time the source was activated, the ship was severely shaken which threatened its structural integrity.
A number of marine sources are known which rely upon a pair of opposing pistons mounted in a sleeve that are initially in intimate contact. The pistons are, by hydraulic or electro-hydraulic means, caused to abruptly expand outwardly from each other, against a volume of water, to radiate a wavefield in the water. The wavefield may be periodic, impulsive or implosive. Typical of such sources is described in U.S. Pat. No. 4,153,135, issued May 8, 1979 to John Bouyoucos for an APPARATUS FOR THE GENERATION OF ACOUSTIC SIGNALS IN MARINE ENVIRONMENTS. Several similar types of marine sources are illustrated in another Bouyoucos U.S. Pat. No. 4,207,962, issued Jun. 17, 1980.
Underwater acoustic sources towed through the water experience hydrodynamic drag which slows progress and increases operational expenses. Submerged mechanical acoustic sources also require watertight integrity of the components, both mechanical and electrical, which increases production and maintenance expenses. Furthermore, most submerged vibratory acoustic sources require that the pressure of the interior of the device be equalized with the ambient water pressure in order for the device to operate properly. A significant problem with underwater towed marine sources is the first ghost reflection from the water surface, which produces notches in the farfield power spectrum.